In recent years, small copiers that can swiftly form a great number of high-quality images and can maintain that ability over a long period of time are in high demand; however, not all recent fast-copiers have been successfully downsized. That is partially because the required space for housing collected toner particles remaining after transferring toner images is large in copiers. On the other hand, collecting toner particles is important in terms of the environment, and handling the remaining toner particles has greatly gathered concerns. The forementioned problems can be solved by reusing the remaining toner particles in image developing. By this means, small and fast-copiers that enable to reduce their environmental load can be successfully achieved. As reusing the collected toner particles enables to produce a greater number of copies on the same amount of supplied toner than those not reusing them, such fast-copiers have great economical advantages.
Many attempts have been made to collect and reuse the remaining toner particles. Unfortunately, those attempts cannot maintain ability of stably forming high quality images over a long period of time. This is because image quality and image density degrade and chances of other problems being caused increases at every copy produced in such systems.
Patent Literature 1 proposes collecting and reusing toner particles whose particle size distribution is adjusted in a certain range so that it enables to form high quality images over a long period of time. In the proposed toner, 90% by mass or more of toner particles have a diameter from D(3√2)−1 to 3√2D, and 5% by mass or less have a diameter smaller than D(3√2)−1, where D is the volume average particle diameter of the particles. That proposed toner, exclusively used in two-component developments, contains a small proportion of very small toner particles so that it has advantages that toner scattering as well as fogging (which commonly occurs where toner particles are collected to be reused) can be prevented. Unfortunately, the proposed toner cannot provide high-resolution images. This is because it contains insufficient proportion of small toner particles for forming the fine images.
Patent Literature 2 proposes a toner having another specific particle size distribution; however, carrier spent is yet to be decreased and the occurrence of fogging is yet to be prevented in the proposed toner.
On the other hand, toners containing a large amount of fine particles for forming high-resolution images have some disadvantages when removing remaining particles of such toners from the surface of a photoconductor after the forming of images. One of the disadvantages arises when such toner is used in a system where a cleaning blade is used as means to clean the surface of the photoconductor after the forming of an image. Fine particles which have not transferred, or particles remaining on the photoconductor, are hardly removed with the blade.
Another disadvantage of such toner is that a wax and inorganic particulates are easily detached from the toner particles, attached on the photoconductor. The wax is internally or externally added to the toner in order to improve its releasing property. The inorganic particulates are added to the toner in order to improve its flowability. In smaller toner particles, the proportion of those additives increases in the particles, thus using such smaller particles tends to cause greater amount of such additives to adhere on the photoconductor.
An example of a cleaning unit for removing attached substances from the photoconductor is found in Patent Literature 3. The proposed cleaning unit contains a cleaning blade and a cleaning roller whose surface is covered with an abrasive. Unfortunately, as the abrasive particles covering the surface of the roller easily come off, the proposed technique has difficulty in maintaining its cleaning capability over a long period of time. Another cleaning unit proposed in Patent Literature 4 contains a cleaning blade provided with glued abrasive particles at its edge. One of the disadvantages is that removing both the remaining toner particles and attached substances at the same time is significantly difficult using that blade. Another disadvantage is that the abrasive particles easily come off from the edge.
As described above, removing attached substances from the surface of the photoconductor with such a conventional cleaning blade or a conventional cleaning unit containing such cleaning roller has not achieved a satisfactory result. As a result, unremoved attached substances cause filming when they mainly consist of wax. They degrade image quality over time when they mainly consist of inorganic particulates which serve as cores of growing attached substances.
For the above-stated reason, the inventors of the present invention proposed a cleaning unit found in Patent Literature 5. It suggests using two different blades, a first blade and a second blade, where the second blade is a sanding blade composed of a base and an abrasive particle-containing layer. Around a photoconductor, the first and second cleaning blades are provided at the upper stream and down stream, respectively, of the rotation direction of the photoconductor. Although the proposed cleaning unit can effectively remove remaining toner particles and attached substances from the surface of the photoconductor, it is still not an effective means for removing fine toner particles having a narrow particle size distribution. Thus, cleaning units that can effectively remove such toner particles have been highly demanded.
[Patent Literature 1]: Japanese Patent Application Laid-Open (JP-A) No. 02-157765
[Patent Literature 2]: Japanese Patent (JP-B) No. 2896826
[Patent Literature 3]: JP-A No. 10-111629
[Patent Literature 4]: JP-A No. 2001-296781
[Patent Literature 5]: JP-A No. 2004-117465